Fix a performance bug in the multicore gc version. In hvc files, should not reserve...

Fix a performance bug in the multicore gc version.  In hvc files, should not reserve more than 16M for a core's stack+heap.  Otherwise the performance would be greatly affected.  It is because that all the reserved memory in hvc files will take *wired* DTLB entries.  The more these entries are reserved, the less entries the program can use for other memory and thus the more DTLB miss would happen during execution.  Also fixed some small bugs that break non-gc version and add codes for DTLB flushing